In general, an optically biaxial film is produced by biaxially stretching a film obtained from a polymer (see, for example, JP-A-2-264905 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). However, in recent years, a method of obtaining a biaxial film by using a biaxial liquid crystal has been proposed. The biaxial film using a biaxial liquid crystal is advantageous in that the film thickness can be made very small as compared with biaxial films conventionally used in many cases. Therefore, use of a biaxial liquid crystal for the biaxial film is a very useful technique for realizing thinning, lightweighting or the like of a device. As for such a technique, for example, a method of obtaining a biaxial film by monoaxially stretching a polymer liquid crystal compound of expressing SCA phase which is one of biaxial liquid crystal phases, has been reported (see, for example, JP-A-11-60972). However, the film obtained by utilizing the stretching is very bad in the dimensional stability and often suffers from a problem that the optical performance is readily changed by humidity, heat or the like.
On the other hand, a technique where a biaxial film using a biaxial liquid crystal compound is produced by not using stretching at all has been also reported (see, for example, JP-A-2002-6138). However, in this report, a functional group for fixing the orientation is not positively introduced into the biaxial liquid crystal compound and therefore, this technique has a problem that the film is readily scratched due to low hardness or the orientation is disordered in aging to lose the biaxial alignment. In order to eliminate such a problem of hardness or orientation disorder, a method of fixing the orientation is considered and examples thereof include a method of introducing a polymerizable group. An optical film using a biaxial liquid crystal compound having introduced thereinto such a polymerizable group has been reported (see, for example, JP-A-2002-174730). However, as stated in this report, when a polymerizable functional group is introduced into the biaxial liquid crystal compound, hybrid (not biaxial) alignment results. Furthermore, the liquid crystal compound molecule forms a high pre-tilt angle at the air interface and this causes orientation disorder. Therefore, it is usually very difficult to obtain a biaxial film having a high strength by using a biaxial liquid crystal. In addition, even if a polymerizable group is introduced into the liquid crystal compound, a film having a high strength cannot be always obtained.